Which property holds for a harmonic function f(x,y)f(x, y)f(x,y)?
fxx=fyyf_{xx} = f_{yy}fxx=fyy
fxx+fyy=0f_{xx} + f_{yy} = 0fxx+fyy=0
fxx+fyy=1f_{xx} + f_{yy} = 1fxx+fyy=1
fxy=0f_{xy} = 0fxy=0